The present invention relates generally to diffusion coating treatment for various metal workpieces, and more particularly to a new and improved method to modify the grain structure of a diffusion coated workpiece by a process involving nitriding.
In diffusion coating treatments of carbon and Cr--Mo steels, a phase transformation takes place from ferrite (a body-centered cubic structure) to austenite (a face-centered cubic structure) when the substrate is heated to typical diffusion coating temperatures. As the surfaces of the substrate are enriched with Cr (along with any other elements which may be present in the diffusion coating, including but not limited to Si and Al), the surface layer of the substrate is transformed back to ferrite at the coating temperatures while the alloy core remains as austenite. The resulting microstructure of the coating layer is always columnar (i.e., the grain boundaries have the same depth as the coating layer and form perpendicularly to the surface of the substrate) because directional solid-state diffusion is involved.
The nucleation rate of the coating is relatively slow compared to grain growth during diffusion coating, resulting in large columnar grains within the diffusion coating layer. After the coating treatment, the core of the coated parts transforms back to ferrite by means of nucleation and growth when the substrate cools from typical diffusion coating temperatures, whereas the coating layer itself undergoes no phase transformation during this time. Consequently, the ferritic surface of the coated workpiece (where the diffusion coating layer is created) retains a columnar grain structure.
Such a columnar grain structure makes the coated products susceptible to surface-induced cracking. Furthermore, the grain boundaries act as preferential sites for unwanted carbides to form, e.g., M.sub.23 C.sub.6. Specifically, the precipitation of carbides at the columnar grain boundaries reduces the ductility of the coating and allows localized corrosion attack to take place (i.e., a corrosion mechanism sometimes referred to as "sensitization"). Another disadvantage of a columnar grain structure is that the large columnar grains may possess relatively low hardness, resulting in a soft surface on the coated parts.
Thus, columnar grain structure in diffusion coatings are suspectible to failures when used in various applications. Accordingly, efforts have been made to improve the diffusion coating performance by modifying the diffusion-coating microstructure from columnar to primarily equiaxed.
Heat treatment has been employed to modify the microstructure of alloys that possess different crystalline structures at different temperatures. For example, the crystalline structure of carbon and Cr--Mo steels can be transformed from face-center cube (fcc) to body-center cube (bcc) when the materials are cooled to below approximately 1674.degree. F. (912.degree. C.). As phase transformation occurs, the microstructure is altered via recrystallization and growth of the new phase in the alloy, thereby improving the mechanical properties of the steels. The hardness of an alloy can also be improved by tailoring the grain size of the new phase formed. Thus, an alloy that can be hardened simply by a heating cycle is often referred to as "hardenable."
However, some alloys, such as stainless steels and nickel-base alloys, possess the same crystalline structure throughout the entire temperature range of interest. As a result, no phase transformation can take place by varying the temperature alone. Instead, the implementation of cold working, followed by heat treatment, is necessary to alter the grain structure of these alloys. This group of alloys is classified as "non-hardenable."
Diffusion coatings produced on steels are non-hardenable. Therefore, the microstructures of such diffusion coatings can only be modified by a combination of cold working and heat treatment. However, the use of cold working is impractical for diffusion-coated parts because cold working is prone to damaging the coating and reducing its thickness, thereby defeating the intended purpose of the coating. Furthermore, the amount of cold working necessary to initiate recrystallization and growth in the coating layer often causes significant deformation to the coated parts, such that deformation of many coated components, including boiler tubes, makes them unusable and unacceptable for their intended purpose. With these limitations in mind, the traditional method to modify the grain structure of non-hardenable alloys cannot be directly applied to diffusion coatings. Consequently, development of an alternative grain-modifying process for diffusion coatings is needed.